Elton Simpson, who was killed while attempting to attack the event, had a series of social media exchanges with Mohamed Abdullahi Hassan, an American-born jihadist and "mysterious ISIS recruiter" who has been living in Somalia since 2007.

Hassan, who goes by the nickname "Miski," was part of an initial wave of Minnesota-based youth who traveled to the Horn of Africa to fight alongside Al Shabaab, a jihadist group that initially formed to oppose the Ethiopian invasion of Somalia in 2006. Hassan left for Somalia in 2008 at the age of 17, joining an organization that rapidly morphed into one of the world's most successful recruiters of foreign jihadists.

At the time Hassan arrived, Shabaab largely consisted of fighters that had been members of the Islamic Courts Union, a fundamentalist Islamic political movement that the Ethiopian invasion had removed from power. In Somalia's stateless vacuum, Shabaab was able to create an extensive safe haven for foreign fighters and to develop one of Africa's most dangerous terrorist groups. The group's foreign connections allowed Shabaab to claim a notable jihadist milestone: the first American jihadist suicide bomber in history carried out his attack on Shabaab's behalf, in 2011.

FBI photo of Hassan

Shabaab officially pledged allegiance to Al Qaeda in 2012, and though the group had seen its territory reduced and much of its major leadership killed, it retains startling operational capabilities. On April 2nd, Shabaab killed 147 college students during an attack on a university in eastern Kenya.

According to short profiles from Minnesota Public Radio and the New York Times, Hassan was devoutly religious, and left for Somalia at the age of 17, when he was only one year away from graduating high school. He was determined to join the fight in Somalia, but only made it there on his second try: Hassan and an accomplice had previously attempted to purchase tickets to Africa but a mosque volunteer had caught wind of their plans and stopped them from leaving.

The U.S. Department of State's Rewards for Justice Program is offering rewards for information on four key leaders of the terrorist group Islamic State of Iraq and the Levant (ISIL). The Secretary of State has authorized rewards of up to $7 million for information on ‘Abd al-Rahman Mustafa al-Qaduli; up to $5 million each for information on Abu Mohammed al-Adnani and Tarkhan Tayumurazovich Batirashvili; and up to $3 million for information on Tariq Bin-al-Tahar Bin al Falih al-‘Awni al-Harzi.

Established in 2004 as “al-Qaida in Iraq” and later known as the “Islamic State of Iraq,” ISIL has recruited thousands of followers from across the globe to fight in Iraq and Syria, where ISIL members continue to commit gross, systematic human rights abuses, including mass executions, persecution of individuals and entire communities on the basis of their identity, killing and maiming of children, rape, and numerous other atrocities.

In April 2013, ISIL’s current leader Abu Bakr al-Baghdadi, also known as Abu Du’a, publicly declared that the Islamic State of Iraq was operating under the moniker of ISIL. ISIL has since asserted publicly that it is the true inheritor of Usama bin Ladin’s legacy.

‘Abd al-Rahman Mustafa al-Qaduli is a senior ISIL official who rejoined ISIL following his release from prison in early 2012. He traveled to Syria where he has worked with an ISIL network. He originally joined al-Qaida in Iraq (AQI) in 2004 and served as AQI leader Abu Musab al-Zarqawi’s deputy and as AQI emir of Mosul, Iraq. The U.S. Department of the Treasury designated al-Qaduli as a Specially Designated Global Terrorist pursuant to Executive Order 13224 on May 14, 2014.

Abu Mohammed al-Adnani, whose birth name is Taha Sobhi Falaha, is a senior leader of and official spokesman for ISIL. He is the main conduit for the dissemination of ISIL messages, including its declaration of ISIL’s creation of an Islamic caliphate. In public statements, al-Adnani has repeatedly called for attacks against Westerners and has vowed “defeat” for the United States. The U.S. Department of State designated al-Adnani as a Specially Designated Global Terrorist on August 18, 2014.

Tarkhan Tayumurazovich Batirashvili has served as a senior ISIL commander and Shura Council member. Batirashvili has overseen an ISIL prison facility in al-Tabqa where ISIL possibly held foreign hostages, has worked closely with ISIL’s financial section, and has managed ISIL operations in the Manbij area of Syria. In May 2013, he was appointed ISIL’s northern commander of operations in Syria’s Aleppo, al-Raqqah, Latakia, and northern Idlib provinces. The U.S. Department of the Treasury designated Batirashvili as a Specially Designated Global Terrorist on September 24, 2014.

Tariq Bin-al-Tahar Bin al Falih al-‘Awni al-Harzi was one of the first terrorists to join ISIL and has served as an ISIL official operating in Syria. He has helped to raise funds from Gulf-based donors for ISIL and has recruited and facilitated the travel of ISIL fighters. He was named ISIL’s leader for the border region between Syria and Turkey. As of late 2013, al-Harzi was chief of ISIL’s suicide bombers, overseeing ISIL’s suicide bomber facilitation pipeline. Al-Harzi also has procured and shipped weapons from Libya and Syria for ISIL operations in Iraq. On September 24, 2014, the U.S. Department of the Treasury designated al-Harzi as a Specially Designated Global Terrorist.

More information about these individuals is located on the Rewards for Justice website at www.rewardsforjustice.net. We encourage anyone with information on these individuals to contact the Rewards for Justice office via the website, e-mail (info@rewardsforjustice.net), phone (1-800-877-3927), or mail (Rewards for Justice, Washington, D.C., 20520-0303, USA). All information will be kept strictly confidential.

The Rewards for Justice program is administered by the U.S. Department of State's Bureau of Diplomatic Security. Since its inception in 1984, the program has paid in excess of $125 million to more than 80 people who provided actionable information that put terrorists behind bars or prevented acts of international terrorism worldwide. Follow us on Twitter at https://twitter.com/Rewards4Justice.